Indirectly heated cathode



Oct. 9, 1934. T. M. SHRADER INDIRECTLY HEATED CATHODE Filed July 7, 1935 |NVE NT OR .7522) M. SAW/IDEA D W w ATTORNEY Patented Get. 9, 1934 INDIRECTLY HEATED CATHODE Terry M. Shrader, West Summit, N. J., assignor to'Radio Corporation of America, a corporation of Delaware Application July 7, 1933, Serial No. 679,271

' 4 Claims. (CL 25027.5)

My invention relates toc'athodes for electron discharge devices and more particularly to the heater element of such cathodes.

An object of the invention is to provide a l 5 heater of great mechanical strength and capable of withstanding temperatures of 1800 C. or more without distortion.

A well known type of heater used in radio tubes having an indirectly heated unipotential .m cathode comprises an insulating core rod of refractory insulation, such as Alundum, and a helical heater coil wound on the core rod. For satisfactory operation of the radio tube in which this heater is used, the impurities should be expelled from the heater wire and insulation by firing the heater assembly at a high temperature, such as 1800 C. When the heater assembly is fired at such high temperatures the Alundum core rod becomes fragile, often breaking when 23 inserted in the heater coil, and also after insertion while the coil is being handled for mounting in the tube and, in addition, shrinks materially, often twisting into odd shapes which destroy its usefulness.

In a preferred embodiment of the invention the core rod for the heater is of a metal, such as tungsten or molybdenum, which will not soften or weaken at temperatures of about 1800 C. The metal core rod has a thin coating of refractory insulation and the heating coil, preferably a double helical coil of tungsten wire, fits rather snugly on the insulated core rod. The heater wire is covered with refractory insulation to prevent electrical contact between it and the oathode which it heats. The cathode may be a cylindrical sleeve of nickel with an exterior coating of barium and strontium oxides or similar electron emitting material.

For a better understanding of my invention, so reference may be had to the following description and the accompanying drawing, in which:

Fig. 1 is a view, partly in longitudinal section, of a cathode made in accordance with my invention;

Fig. 2 is a cross-sectional view along the line 2-2 of Fig. 1.

In the embodiment of the invention shown in Fig. 1, the heater comprises a metal core rod 1 having a thin coating of insulation 2 over which a double helical heater coil 3 is fitted. The core rod is preferably a wire of tungsten or molybdenum, metals which have the desired mechanical strength and rigidity at very high temperatures. The heater coil 3 is preferably of tungsten wire. The insulating coating 2 electrically insulates the coil 3 from the metallic core rod 1. V This insulating coating, preferably of some refractory oxide such as Alundum, ad.- heres firmly to the metal core rod, is too thin to be self-supporting, and hence cannot expand and contract independently of the core rod, does not possess suflicient mass to increase appreciably the heating up time of the coil, and will withstand firing at 1900" C. or higher without shrinking, twisting, or peeling. The heater wire 3 is covered with an Alundum or similar refractory oxide layer 4 of approximately the same thickness as the coating 2 on the core rod 1.

The heater shown in Fig. 1 may, for example, be made as follows: The core 1, preferably a molybdenum or wrought tungsten Wire of approximately 20 mils diameter, is sprayed with a suspension of aluminum oxide to form an in sulating coating approximately 5 mils thick. The sprayed and coated core rod is then fired in a hydrogen atmosphere to about 1900 C. for about ten minutes, causing the alundum coating to harden and adhere firmly to the core rod. The heater coil 3, of wrought tungsten wire wound into a double helix coil on a winding machine, is slipped over the fired insulated core rod which fits snugly inside the coil. The heater assembly is then sprayed with aluminum oxide in suspension to cover the coil 3 and the insulated core rod with a layer or covering about 5 mils thick, and then fired in a hydrogen atmosphere to about 1900 C. for approximately ten minutes. This procedure produces a very straight, compact, and rigid heater assembly, capable of withstanding exceptionally high temperatures without appreciable distortion and of great mechanical strength. Such a heater is highly desirable in view of the stringent requirements for radio tubes as to gas and heater cathode leakage characteristics. This heater assembly is heated to such a high temperature that gases and other impurities are expelled, whereby the residual gas due to the heater is exceptionally low, and the heater cathode leakage is exceptionally small.

100 Fig. 2 is a cross-section through the cathode of Fig. 1, showing the metal core rod 1 with a thin firmly adherent coating 2 of aluminum oxide, a tungsten heater wire 3 fitted over and resting on the insulated core rod, a thin covering of aluminum oxide 4 on the heater coil 3, and a cathode sleeve 5 having a coating 6 of electron-emitting material such as barium and strontium oxides.

While the preferred embodiment has been disclosed, the invention admits of various changes which are limited in scope only by the appended claims.

What is claimed as new is:-

1. In an equipotential cathode for an electron discharge device, the combination with an electron emitting cathode sleeve, of a heater within said sleeve comprising a core rod of wrought metal rigid at1800 C., an adherent refractory insulating coating on said core rod in thickness less than the radius of said core rod, a heating coil of the same wrought metal surrounding and supported by said insulated core rod,,and an core rod and of approximately the same thickness as the insulating coating on said tungsten core.

3. A heater assembly for an electron discharge device comprising a wrought tungsten core rod having an adherent coating of refractory insulation of a thickness less than the radius of said core rod and too thin to be self-supporting, a double helical heating coil of tungsten wire surrounding and supported on said insulated core rod, and a layer of refractory insulation covering and adherent to said heating coil and said coating of insulation on said core rod and of about the same thickness as the insulating coating on said core rod. I 4. A heater assembly for an electron discharge device comprising a tungsten core rod about 20 mils in diameter and having an adherent coating of aluminum oxide about 5 mils thick, a double helical heating coil of tungsten wire on said in- -TERRY M. SHRADER. 

